Classification of restriction Endonucleases

 

The restriction endonucleases work in coordination with the methylases which is in turn referred as restriction-modification system. This system helps them to distinguish between the foreign DNA and the native DNA.  Each of these enzymes has their own distinct mechanisms of action on the target double-stranded palindrome sequences. Hence based on their mode of action, they are classified into three classes.

i.        Type I:            

These enzymes cleave the DNA sequences 1000 base pairs away from the 5’ end of the restriction site or recognition sequences. They are bifunctional enzymes which have both methylase and restriction activities. However, they have three subunits namely R, M and S (R for restriction, M for methylation and S for specific target site). The length of the predefined palindrome recognition sequences range from 10 to 15 base pairs. In addition to this the enzymes require ATP activity as energy source.

E.g.:  Eco K and Eco B

ii.      Type II:            They are discovered by Hamilton Smith, which are capable of cleaving the target DNA within the restriction sites. The enzyme system possesses separate enzymes for modification and restriction i.e. Methylation subunit exists separately from the restriction subunit. Because of this added advantage these enzymes are the most widely used endonucleases in r-DNA technology and gene cloning. Moreover the length of the predefined restriction site is 4 to 6 base pairs, which is smaller when compared with the type I and II enzymes.

See Also: Nomenclature of Restriction Endonuclease Mechanism of Restriction Endonuclease

E.g.:  Eco R I, Hind II and Ban H I.

iii.    Type III:          

These enzymes cleave the DNA sequence 26 base pairs away from the restriction site. They are also bifunctional but have only two subunits namely R and MS (R for restriction and MS for modification and specific recognition). Here again the length of the palindromes that are predefined as recognition sites range from 10 to 15 base pairs. However, this class of endonucleases requires ATP activity as energy source.

E.g.: Eco P I and Eco 15.